The invention includes a high repetition rate (repetition rate xe2x89xa74 kHz) fluoride excimer laser with magnesium fluoride laser optics for transmitting and controlling the UV xcex photons produced by the laser. The magnesium fluoride high repetition rate UV excimer laser optics provide for improved reliability in the operation of xe2x89xa74 kHz high repetition rate laser systems. The magnesium fluoride containing high repetition rate laser provides for the production of a high laser power (xe2x89xa710 mJ) output at a high repetition rate (xe2x89xa74 kHz) for a long laser system operation time ( greater than 500 million pulses, preferably xe2x89xa7900 million pulses) with a magnesium fluoride laser optics reliability that avoids catastrophic damage of the laser optics and related catastrophic laser system failure. In a preferred embodiment the magnesium fluoride laser optics are utilized in a UV xcex less than 200 nm ArF excimer laser with a 4 kHz repetition rate and an output power of 10 mJ.
The manufacture of semiconductor chips can be achieved using excimer lasers as a light source. Krypton Fluoride lasers with a wavelength of xcx9c248 nm were the first excimer lasers to be used. As the semi-conductor chip technology has evolved, lasers of higher energy and higher repetition rate are required. One such excimer laser is known as Argon Fluoride emitting at xcx9c193 nm. For various applications it is preferable to have such a laser with a repetition rate of 4 kHz. In both Krypton Fluoride lasers and Argon Fluoride lasers, the preferred optical material for windows, beamsplitters, output couplers and line narrowing prisms has been calcium fluoride. At high repetition rates such as 4 kHz it has been observed that the calcium fluoride chamber windows suffer catastrophic damage in a relatively short time scale [less than 500 million pulses for a 4 kHz, 193 nm ArF laser with an output power of 10 mJ.] The damage to the window can be as severe as cracking but at a minimum results in wavefront distortion and increased birefringence. Changing of the windows results in an increased cost of operation of the laser and therefore increased cost of ownership for the chip manufacturer.
Calcium fluoride is a cubic material, optically isotropic with excellent transmission in the VUV, UV, visible and IR parts of the spectrum. When calcium fluoride is illuminated with 193 nm laser light, the material emits photons in the near UV. The observed fluorescence in turn means energy is being absorbed. As the band gap of calcium fluoride is much larger than the energy of the laser photons it is surmised that there is a multi-photon absorption process. It is proposed that the center created by multi-photon absorption can itself absorb further photons and this can lead to heating and consequently material degradation. If the lifetime of the absorption center is sufficiently long then at 4 kHz heating can occur whereas at lower repetition rates, heating may not be observed.
Magnesium fluoride is a tetragonal material and therefore optically anisotropic. As such it has not been a favored material for lithographic lasers because of the importance of polarization within the manufacturing process. Magnesium fluoride has a wider bandgap and a lower quench temperature for self trapped excitonic emission.
The invention includes a xe2x89xa74 kHz repetition rate argon fluoride excimer laser system for producing an UV wavelength 193 nm output. The xe2x89xa74 kHz repetition rate argon fluoride excimer laser system includes an argon fluoride excimer laser chamber for producing a 193 nm discharge at a pulse repetition rate xe2x89xa74 kHz. The xe2x89xa74 kHz repetition rate argon fluoride excimer laser chamber includes at least one magnesium fluoride crystal optic window for outputting the 193 nm discharge as a 4 kHz repetition rate excimer laser 193 nm output with the magnesium fluoride crystal optic window having a 255 nm induced absorption less than 0.08 Abs/42 mm when exposed to 5 million pulses of 193 nm light at a fluence xe2x89xa740 mj/cm2/pulse and a 42 mm crystal 120 nm transmission of at least 30%. That is, the invention includes a xe2x89xa74 kHz repetition rate argon fluoride excimer laser system in which the laser chamber has at least one window made from a single crystal of magnesium fluoride, and such window, after exposure to 5 million pulses of 193 nm light having a fluence of 40 mJ/cm2/pulse, has an absorbance of less than 0.08 Abs per 42 mm path length when measured at 255 nm and a 120 nm transmission of at least 30% through a 42 mm path.
The invention includes a xe2x89xa74 kHz repetition rate excimer laser system for producing an UV wavelength xcex less than 200 nm output. The xe2x89xa74 kHz repetition rate excimer laser system for producing an UV wavelength xcex less than 200 nm output includes
an excimer laser chamber for producing an UV wavelength xcex less than 200 nm discharge at a pulse repetition rate xe2x89xa74 kHz. The excimer laser chamber includes at least one magnesium fluoride crystal optic window for outputting the xcex less than 200 nm discharge as a xe2x89xa74 kHz repetition rate excimer laser xcex less than 200 nm output with the magnesium fluoride crystal optic window having a 255 nm induced absorption less than 0.08 Abs/42 mm when exposed to 5 million pulses of 193 nm light at a fluence xe2x89xa740 mj/cm2/pulse and a 42 mm crystal 120 nm transmission of at least 30% and a 200 to 210 nm range absorption coefficient  less than 0.0017 cmxe2x88x921.
The invention includes a xe2x89xa74 kHz repetition rate fluoride excimer laser magnesium fluoride crystal optic for transmitting a xe2x89xa74 kHz repetition rate fluoride excimer UV wavelength xcex less than 200 nm output. The xe2x89xa74 kHz repetition rate fluoride excimer laser magnesium fluoride crystal has a 255 nm induced absorption less than 0.08 Abs/42 mm when exposed to 5 million pulses of 193 nm light at a fluence xe2x89xa740 mj/cm2/pulse and a 42 mm crystal 120 nm transmission of at least 30%.
The invention includes a xe2x89xa74 kHz repetition rate fluoride excimer laser magnesium fluoride crystal optic window for transmitting a xe2x89xa74 kHz repetition rate fluoride excimer UV wavelength xcex less than 200 nm output. The xe2x89xa74 kHz repetition rate fluoride excimer laser crystal optic window is comprised of a magnesium fluoride crystal with a 255 nm induced absorption less than 0.08 Abs/42 mm when exposed to 5 million pulses of 193 nm light at a fluence xe2x89xa740 mj/cm2/pulse and a 42 mm crystal 120 nm transmission of at least 30% and a 200 to 210 nm range absorption coefficient  less than 0.0017 cmxe2x88x921.
The invention includes a xe2x89xa74 kHz repetition rate argon fluoride excimer laser crystal optic for transmitting an UV wavelength 193 nm argon fluoride excimer laser xe2x89xa74 kHz repetition rate output with the laser crystal optic comprised of a magnesium fluoride crystal with a 255 nm induced absorption less than 0.08 Abs/42 mm when exposed to 5 million pulses of 193 nm light at a fluence  greater than 40 mj/cm2/pulse and a 42 mm crystal 120 nm transmission of at least 30%.
The invention includes a xcex less than 200 nm optical fluoride crystal for transmitting a UV wavelength xcex less than 200 nm with the xcex less than 200 nm optical fluoride crystal comprised of a magnesium fluoride crystal with a 255 nm induced absorption less than 0.08 Abs/42 mm when exposed to 5 million pulses of 193 nm light at a fluence xe2x89xa740 mj/cm2/pulse and a 42 mm crystal 120 nm transmission of at least 30%, and a Fe contamination level less than 0.17 ppm Fe by weight, a chrome contamination level less than 00.08 ppm chrome by weight, a copper contamination level less than 0.04 ppm copper by weight, a cobalt contamination level less than 0.04 ppm cobalt by weight, an Al contamination level less than 0.9 ppm Al by weight, a nickel contamination level less than 0.04 ppm nickel by weight, a vanadium contamination level less than 0.04 ppm vanadium by weight, and a lead contamination level less than 0.04 ppm lead by weight and a 200 to 210 nm range absorption coefficient  less than 0.0017 cmxe2x88x921. Preferably the Fe contamination level is less than 0.15 ppm Fe by weight, the chrome contamination level is less than 0.06 ppm chrome by weight, the copper contamination level is less than 0.02 ppm copper by weight, the cobalt contamination level is less than 0.02 ppm cobalt by weight, the Al contamination level is less than 0.7 ppm Al by weight, the nickel contamination level is less than 0.02 ppm nickel by weight, the vanadium contamination level is less than 0.02 ppm vanadium by weight, and the lead contamination level is lass than 0.02 ppm lead by weight. That is the invention includes an optical fluoride crystal comprised of a magnesium fluoride crystal that when exposed to 5 million pulses of 193 nm light at a fluence xe2x89xa740 mj/cm2/pulse has an induced absorption of less than 0.08 Abs per 42 mm path, and a 120 nm transmission of at least 30% through a 42 mm crystal; and contaminants at levels as specified above in this paragraph.
The invention includes a xe2x89xa74 kHz repetition rate argon fluoride excimer laser crystal for transmitting an UV wavelength 193 nm argon fluoride excimer laser xe2x89xa74 kHz repetition rate output with the laser crystal comprised a magnesium fluoride crystal with a 255 nm induced absorption less than 0.08 Abs/42 mm when exposed to 5 million pulses of 193 nm light at a fluence xe2x89xa740 mj/cm2/pulse and a 42 mm crystal 120 nm transmission of at least 30% and a Fe contamination level less than 0.17 ppm Fe by weight, a chrome contamination level less than 00.08 ppm chrome by weight, a copper contamination level less than 0.04 ppm copper by weight, a cobalt contamination level less than 0.04 ppm cobalt by weight, an Al contamination level less than 0.9 ppm Al by weight, a nickel contamination level less than 0.04 ppm nickel by weight, a vanadium contamination level less than 0.04 ppm vanadium by weight, and a lead contamination level less than 0.04 ppm lead by weight. Preferably the Fe contamination level is less than 0.15 ppm Fe by weight, the chrome contamination level is less than 0.06 ppm chrome by weight, the copper contamination level is less than 0.02 ppm copper by weight, the cobalt contamination level is less than 0.02 ppm cobalt by weight, the Al contamination level is less than 0.7 ppm Al by weight, the nickel contamination level is less than 0.02 ppm nickel by weight, the vanadium contamination level is less than 0.02 ppm vanadium by weight, and the lead contamination level is less than 0.02 ppm lead by weight.
The invention includes magnesium fluoride crystal optics in 4 kHz lithographic, xcx9c193 nm (centered about 193 nm) excimer lasers with the magnesium fluoride crystals suitably oriented to minimize the effects of the intrinsic birefringence of magnesium fluoride. The magnesium fluoride crystal optics 4 kHz lithographic 193 nm excimer laser provide the benefits of longer lifetime and improved performance of the optics leading to a reduced cost of ownership of the laser. FIG. 1 shows a lithographic excimer laser system in accordance with the invention with magnesium fluoride crystal laser chamber windows 20 and magnesium fluoride crystal line narrowing module beam expanding prisms 30. FIG. 2 shows an excimer laser chamber in accordance with the invention with magnesium fluoride crystal laser chamber windows 20.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) of the invention, and together with the description serve to explain the principles and operation of the invention.